1. Field of the Invention
This invention relates to engaging elements for drag type rotary drill bits. More particularly, the invention is the shape of the contact face portion of a drill bit engaging element upon which a super hard abrasive material such as diamond, cubic boron nitride, a combination of the two, or similar material is deposited.
Throughout the following disclosure the term "abrasive" is intended to cover all abrasive materials including but not limited to synthetic diamond, cubic boron nitride, polycrystalline diamond, polycrystalline cubic boron nitride, and combinations thereof. The type of abrasive material placed on the contact face of the drill bit engaging element does not form any part of this invention, so long as it is harder and more wear-resistant than the material forming the body of the drill bit engaging element.
2. Prior Art
The application of abrasive material such as sintered polycrystalline diamond compacts to the contact portion or working surface of the engaging element in rotary type drag bits has extended the life expectancy of the drag bits and also allowed the design and development of rotary type drag bits with more aggressive cutting actions and resultant faster penetrations rates. At present the rotary type drag bits for drilling in either soft or brittle formations include an engaging element with abrasive material deposited on a planar working surface. This planar working surface or contact portion of the engaging element is positioned at a slight negative angle from the perpendicular with respect to the material contacted. Through the rotary action of the bit, the contact portion is dragged or moved across the material. A problem with this planar shape for the contact portion is that the loosened material flows normal to it and is not encouraged in any particular direction. Therefore, the loosened material tends to back up against the contact portion or working surface. Recent bit designs include positioning the engaging elements so that they have a slight side rake in an effort to create a snow plow effect. However, even these new designs are not effective in preventing the material from backing up against the working surface since pressures against the working surface of these designs actually encourage a back-up of material. While in original and the new designs the material will eventually find its way off the outer edges of the working surface, the build-up of material causes numerous detrimental effects.
One detrimental effect resulting from this build-up on the working surface is an increase in wear to the surface caused by the unnecessary and wasteful regrinding of this already loosened material.
Another detrimental effect caused by material backing up on the working surface of the engaging element is a decrease in the initial contact force the engaging element has on new material. When there is a constant layer of ground material backed up on the working surface, it serves as a cushion and actually softens the impact that the engaging element has as it moves into the new material. This is especially critical when drilling in shale or other plastic materials where it is necessary to severely strain the plastic material in order for it to be broken up and removed from the hole.
Yet another detrimental effect of this build-up of material is that it increases the temperatures to which the engaging element is subjected. Higher temperatures are critical when dealing with abrasive material such as polycrystaline diamond since crystal degradation can occur at temperatures as low as 700.degree. C. Any build-up of loosened material on the working surface of the engaging element acts to insulate the engaging element both from the cooling action of the drilling fluid and from the natural cooling action that occurs when the engaging element is continually contacting new material. This insulation is detrimental since it greatly reduces the engaging element's ability to dissipate the heat that is continuously being added to it through the scraping of the distal surface of the contact portion on the bottom of the hole.
The detrimental effect of heat is intensified when drilling in shale or similarly soft formations since under the high pressures and temperatures of drilling the shale becomes tacky or plastic. Accordingly, the build-up of loosened material on the working surface of the engaging element is increased since the tacky material will not readily glide or move off of the working surface as do the small particles which form when a more brittle rock is drilled.
Numerous attempts have been made to alleviate the problem of build-up on the working surface of the engaging element. Several patents disclose drill bit designs which include special hydraulic features such as nozzles, jets, or channels specifically positioned to wash the working surface of the engaging elements and encourage a particular direction of flow for the loosened material. (See U.S. Pat. No. 4,246,977 for Diamond Studded Insert Drag Bit with Strategically Located Hydraulic Passages for Mud Motors issued to James Allen on Jan. 27, 1981; U.S. Pat. No. 4,303,136 for Fluid Passage Formed by Diamond Insert Studs for Drag Bits issued to Harry Ball Dec. 1, 1981; also U.S. Pat. No. 4,334,585 for Insert Retention and Cooling Apparatus for Drag Bits issued to Robert G. Upton June 15, 1982.).
Even without this build-up of material on the face of the engaging element, the planar shape contact portion and the new designs are inefficient with respect to the cooling through hydraulics. Principles of heat transfer and fluid dynamics teach that the convection heat transfer coefficient for a body passing through a fluid varies greatly depending on the shape of the body. Planar faces having fluid flowing normal to them are among the least effective at convective cooling in the fluid. This result is caused in part by the stagnation layer in the fluid that is set up against the working surface.
A recent attempt at facilitating the removal of the material from the working surface of the engaging element is disclosed in the U.S. Pat. No. 4,333,540 in which the contact portion of the bit insert is formed by the intersection of two planar working faces. The cutting face of the contact portion looks like a triangle or wedge. However, there are specific problems which this triangular shape encounters when drilling in shale or other plastic materials. Under the pressure and temperature of drilling, shale tends to become plastic, therefore, although the triangle shape moves the loosened material to the outside of the contact portion of the engaging element, the sharp point of the wedge shaped contact portion will have more of an affect of simply parting the plastic-like shale, tending to form a groove rather than to cause any significant break up of material. This gentle parting action does not produce the strain needed to break up the shale for subsequent removal from the drilling hole.
Another attempt at improving drill bits by changing the contact portion of the drill bit insert is disclosed in U.S. Pat. No. 4,241,798 for Drilling Bits for Plastic Formations issued to Kenneth Jones on Dec. 30, 1980. The Jones patent recognizes the problems encountered in drilling through shale or other plastic-like formations and discloses the need for removing the plastic-like material from the drill hole in such a way as to prevent its reattachment to the bottom or side of the hole. Jones provides an opening in the face of the contact portion of the drill bit insert through which the material is extruded, thereby breaking it into small particles and allowing the drilling fluid to flush the extruded material from the drill hole. The opening in the face of the contact portion reduces the mechanical strength of the contact portion itself which could lead to fracture under the high pressure and temperature conditions experienced during drilling. Furthermore, hard particles can plug the opening in the contact portion and prevent its operation.